Half a century after James Croll, the cyclic nature of the ice ages was proposed yet again, this time by the Serbian engineer and meteorologist Milutin Milankovitch. Using calculations not unlike those of Croll, he proposed that the warmth of the summer, rather than the severity of winter, is the crucial element in determining the course of the ice ages. Cool summers (when the Sun happens to be farther from the Earth in June and July than average, because of eccentricity) allow snow to persist, while warm summers (when the Sun is close to Earth during the months of a high sun angle) remove snow and ice. The leading climatologist of the time, Wladimir Köppen, and his son-in-law, Alfred Wegener, readily supported his ideas. They published a paper that defended Milankovitch's theory of the ice ages.

Despite their support, Milankovitch encountered much the same problem as Croll: he explained features of geologic history that had not been established. His theory was a prediction looking for data to test it. The ice-age deposits that had been studied so far were on the whole rather poorly dated. No one really knew what ages to assign to the variously named ice ages: the "Günz," the "Mindel," the "Riss," and the "Würm." Guesses were made (and were not all that far off), but the precise calculations of orbital changes made by Milankovitch called for a precise fit to well-dated climate history. His scheme covered the last third of what we now call the Pleistocene and the course of climate change over this interval (or any other) was still unknown right into the 1960’s. Pleistocene glacial deposits on land are highly irregular, an hence further detailed studies in the mountains and the plains actually did not advance matters pertaining to cyclicity of the Earth’s orbit. Instead, it was two new developments that brought great progress: the careful study of the record found in deep-sea sediments and the radiometric dating of raised coral deposits, which gave a precise date for the last interglacial.

Sketch of the Kullenberg piston corer. (From a drawing in Kullenberg, 1943)

Deep-Sea Sediment Record

Before one can study record of the ice ages in deep-sea sediments, one has to raise the sediments from the sea floor and do it in such a manner as to minimize disturbance. This breakthrough came with the Swedish Deep-Sea Expedition (1947-1949), a greatly successful circumglobal voyage of the proud four-masted sailing vessel Albatross. The technical innovation that made precise coring was a new kind of device invented by Börje Kullenberg, a marine geologist working in Gothenburg, Sweden who called it a "piston corer." A clever modification of the traditional steel tube principle, the coring tube now fell past a stationary piston at the end of the wire, so that water is expelled from the falling tube above the piston and sediment is admitted from below. To the great delight of the geologists on board, the device allowed the taking of long cores, more than 10 meters in some cases, at many stations all around the world.By the end of the expedition, the Albatross had collected a great number of sedimentary sequences from all ocean basins, with records reaching back between one-half million to one million years. In all, there were about 200 cores with a combined length of one entire mile! Many times during the voyage Kullenberg and his colleagues had had to cope with the depressing sight of bent or broken coring equipment returning on deck from its excursion to the deep sea floor. Unfailingly they went back to work, finding spare parts, cutting, mending, and welding. Anytime the corer hit hard bottom, or landed on a steep slope, chances for success were greatly diminished. The ingredients of success were vigilance and skill in using an echo-sounder in positioning the vessel, dexterity and deftness in controlling the enormous winch, correct timing of the release of the corer hanging on the wire like a bouncing yo-yo, and good luck.With this one supreme effort, the ocean's memoirs regarding the dates of the ice ages (and human evolution) were now available. Detailed and extensive studies on the physical properties, the chemistry, and the microfossil content of the core materials followed, first by expedition’s participants and later by marine geologists in Gothenburg, at Scripps Institution of Oceanography, and at other research facilities. The now famous scientists working on the Albatross material included Gustaf O. S. Arrhenius, Fred B. Phleger, Cesare Emiliani, Frances L. Parker, and Eric Olausson. These pioneers founded the new field of "ocean history," a kind of archeology based on digging up the memories contained in the deep-sea record. The modern name for their field is "paleoceanography."